1. Field of the Invention
The present invention relates to methods and apparatus for using arterial blood pressure to dilate the lumen rather than expanding/cutting mechanical means so as to preserve the sensitive endothelial lining of the artery. More specifically, the present invention relates to production of a temperature profile within the arterial wall that corresponds to the atherosclerotic lesion size, shape, and position so as to necrose (ablate) connective tissue and soften plaque to thereby impart increased flexibility to the arterial
2. Description of Prior Art
Atheroscleroses, also known as hardening of the arteries, is one of the most common types of heart disease. Each year several hundred thousand people die of this disease or of complications relating thereto. By the age of forty, many people have already developed atherosclerotic lesions although no symptoms may appear. Atherosclerosis is a progressive disease wherein fatty, fibrous, calcific, or thrombotic deposits produce atheromatous plaques within the arterial walls as indicated generally in FIG. 11. Moreover, atherosclerosis tends to involve large and medium sized arteries such as the aorta, iliac, femoral, cerebral, as well as the coronary artery.
The atherosclerotic lesions are substantially comprised of plaque and scar tissue. The plaque is typically encapsulated within living connective tissue. Plaque is a heterogenous material, sometimes non-living, that may include calcified, wax-like, fibrotic, fatty, and brittle components. As the lesions within the arterial walls grow in size, the lumen or passageway through the coronary artery may be correspondingly reduced in effective cross-sectional diameter (stenosis). The constricted (stenotic) lumen may then restrict the nutrient blood flow to muscles of the heart. Therefore, atheroscleroses is often a major contributing factor in both acute and chronic heart problems. Also, when the lumen is sufficiently narrowed, the rate of blood flow may be so diminished that an affixed blood clot (thrombus) or circulating blood clot (embolus) may spontaneously occur. Thus, the presence of atherosclerotic plaque not only reduces blood flow to the heart muscle but also is a major predisposing factor in coronary thrombosis.
While pharmacological treatment is often used to treat atherosclerosis, such treatment is sometimes considered to be insufficiently effective for increasing blood flow. Therefore, techniques such as balloon angioplasty (percutaneous transluminal coronary angioplasty) have been developed to mechanically increase the luminal opening
However, mechanical techniques tend to traumatize the artery and often result in restenosis (reclosing of the lumen). Standard balloon angioplasty produces a gross trauma that severely injures the endothelium or lining of the artery. The endothelium is a very fragile layer of cells that performs the very important function of limiting thrombotic processes (See FIG. 12). Unlike other surfaces and materials, when blood cells come in contact with the endothelium there is no tendency to form a clot. The layer of endothelial cells normally covers the internal surface of all vessels, rendering the vessel surface compatible, i.e., non-thrombogenic and non-reactive with blood. If the endothelium is damaged, platelet deposition becomes a problem. Once this layer is injured, it does not normally grow back uniformly and so damage thereof tends to permanently induce thrombotic processes. Injury to the endothelium has been associated with accelerating of atherosclerotic processes and/or restenosis.
At least one patent discussed hereinafter teaches to alleviate, at least to some extent, damage caused to the endothelium by providing a bioprotective layer around the angioplasty balloon to cushion, coat, and insert anti-inflammatory agents prior to opening the balloon. Another patent discussed hereinafter teaches reducing trauma damage to the endothelium by reducing angioplasty balloon pressure using heat and feedback control. However, due to the mechanical forces in balloon angioplasty, damage to the fragile endothelium may still occur. The bioprotective coatings may, according to the patent teachings, also reduce problems created by angioplasty-induced tissue tears or cleavages that can result in blood accumulation along the cleavage planes of arterial wall layers. The thickening caused during such problems may acutely block or occlude the arterial lumen and require emergency interventions such as stents or open heart surgery.
The trauma to the endothelium and to arterial walls from balloon angioplasty often evoke a potent inflammatory response, or restenosis (reclosing of the artery). whereby scar tissue growth and other processes occur to once again reduce the arterial lumen opening. As discussed in some detail below with respect to numerous different techniques, restenosis is probably one of the most significant problems that occurs with existing techniques for dilating or opening the arteries. Restenosis may occur due to numerous processes including the following: clots that grow gradually or contain growth hormones released by plateletes within the clot, clots formed due to exposure to collagen (connective tissue) which is highly thrombogenic, growth hormones released by other cells that cause smooth muscle cells and fibroblasts in the region to multiply, tears in the artery wall that expose blood to foreign material and proteins thereby producing clots, white cells that migrate to the area and lay down scar tissue, growth of new tissue due to any kind of inflammatory response, and/or other factors that may cause the artery lumen to reclose. Several different processes may operate simultaneously to cause restenosis.
After balloon angioplasty procedures, narrowing of the artery by 50% may occur in more than one-half of the patients and about one quarter of the patients may require a repeat procedure by the end of one year. As mentioned above, restenosis (reclosing of the artery) remains a major problem in treatment of atherosclerosis by balloon angioplasty. While significant efforts have been made to dampen the reactive proliferation with a variety of pharmacological and genetic interventions, these interventions have thus far found limited or no successful clinical application.
Another problem with balloon angioplasty relates to the fact that the lesions, or atheromas, within arterial cross-sections are typically asymmetrically distributed. That is to say the lesions often occur on one side, portion, or sector of the artery while an opposite or adjacent sector of the artery is relatively free of the disease. It was initially postulated that the increase in endovascular pressure created with a balloon would somehow selectively crush lesions. However, given that the thick-walled sectors containing atheromas are typically abundant with dense collagen or connective tissue, the lesions may actually be less likely to undergo deformation than the opposite relatively thin-walled, lesion-free sectors. Thus, the maximum trauma produced by balloon angioplasty may well occur in the healthy sector of the artery. Some of the patents discussed hereinafter attempt to reduce, at least to some extent, this problem and related problems by first heating the artery to soften the plaque. However, unless the heating is applied selectively to the atheromas heat damage may occur to the artery and endothelium. As well, mechanical trauma may still occur to the endothelium and/or other arterial components.
Acute closure of the lumen after balloon angioplasty remains the most common cause of failed angioplasty and occurs in approximately 5% of the patients. This complication may be associated with varying degrees of mortality that man exceed 50% when emergency coronary bypass surgery has to be performed.
For the above reasons, balloon angioplasty is often problematic due to restenosis or, less frequently, abrupt closure. Therefore, other catheter techniques for the mechanical widening of the lumen of diseased coronary arteries have been devised, including various tissue cutting techniques. Directional coronary atherectomy (DCA) involves the selective excision and retrieval of atheromatous or proliferative tissue obstructing the arterial lumen. In this procedure, the end of the catheter is provided with a fenestrated metal cylinder within which a piston-like cutter is moved via a torque cable. The cut tissue is recovered in a distal portion of the catheter. As compared with balloon angioplasty, the initial luminal widening achieved by DCA is similar or perhaps slightly better. Again, restenosis is problematical and, as before, the restenosis is probably acerbated by damage to the endothelium.
Rotational atherectomy incorporates a rotary file or burr (brass ellipsoid of varying diameter covered with diamond chips--rotablator) into the distal portion of the catheter. The rotary file or burr is connected to a motor-operated drive shaft. Abrasion of the narrowed artery segments produces micro particles that are allowed to escape into the distal circulation for capillary entrapment. The rotablator has been used for treating heavily calcified lesions, lesions at branch points (`ostial disease`), and lesions involving long arterial segments (`diffuse disease`). Restenosis frequently occurs. However, because this instrument is often used to treat refractory or difficult-to-treat lesions, the results of the treatment are difficult to compare with those of balloon angioplasty. Transluminal extraction catheter atherectomy (TEC) involves use of a catheter with tip-mounted cutting blades. The tissue fragments are recovered by aspirating a flush solution during cutting. As with the other mechanical devices discussed above, restenoses are frequent.
In summary, the exact role of atherectomy or mechanical tissue cutting, devices is not yet completely defined for treatment of atherosclerosis, but it is fair to say that such devices do not overcome the major complication of balloon angioplasty, i.e., restenosis.
Eximer (UV) laser coronary angioplasty systems are the only laser devices currently approved by the FDA for treatment of atherosclerosis. These devices act to remove tissue at the lesion from the artery. Light in the UV range can remove tissue by triggering photochemical reactions (photoablation). However, there is also considerable evidence that UV lasers remove tissue by a thermal mechanism like that of longer wavelengths (vaporization) such as visible, infrared, millimeter wave, and microwave spectra. One characteristic of eximer radiation (308 nm) is that the energy deposition is quite superficial. Accordingly the procedure destroys the superficial cell layer (endothelial cells) and consequently produces a strong inflammatory response as does balloon angioplasty. Not unexpectedly, and in contrast to early claims, UV laser angioplasty does not have a lower restenosis rate than balloon angioplasty. This fact also suggests that tissue deformation, as occurs in balloon and rotablator procedures, is not the major determinant of restenosis because lasers do not forcefully distend artery walls.
Acute coronary closure after balloon angioplasty may be treated in several ways including perfusion balloon angioplasty (prolonged balloon inflation with protective distal coronary perfusion), atherectomy catheters as discussed above, and intracoronary stents. Coronary stents are designed to scaffold artery walls. The use of stents for acute closure is associated with a high initial success rate in maintaining arterial patency.
However, stent therapy suffers from several drawbacks including subacute stent thrombosis, bleeding due to the necessarily aggressive anticoagulation treatment, and restenosis that occurs in 30% to 50% of the cases. The use of stents in non-emergency situations is an experimental procedure.
Another procedures for treating atherosclerosis is coronary artery bypass surgery. However, this procedure does not clearly suggest a better prognosis than balloon angioplasty and has the disadvantage of being a major surgical procedure.
Catheters with 1.2 or 1.7 mm ball tips for ultrasound (195 kHz) destruction of atheromatous lesions in artery walls have been reported.
Experimental use of microwave balloon angioplasty with a gap antenna operating at 2.45 GHz has been reported and the authors concluded that balloon angioplasty combined with microwave heating yielded wider luminal diameters four weeks after intervention. Other uses of heating in combination with balloon angioplasty are discussed in the following patents.
U.S. Pat. No. 4,583,556, issued Apr. 22, 1986. to Hines et al., discloses a microwave applicator with a suggested use for treating cancer. The microwave applicator is said to provide uniform heating without hot spots and includes a first electrical conductor and a second electrical conductor substantially shielding the first conductor in a transmission line configuration. A coil is provided as a third electrical conductor that surrounds an unshielded portion of the first conductor. No particular microwave frequencies are designated.
U.S. Pat. No. 5,496,311, issued Mar. 5, 1996, to Abele et al., discloses an expandable balloon catheter that simultaneously heats the plaque and applies pressure to tissue of the lumen with feedback and software control to thereby significantly reduce the balloon pressure necessary to dilate the artery. The heating is preferably I.sup.2 R (convective) to about 50.degree. C. to 70.degree. C. at the balloon surface, for about 15 to 60 seconds, with balloon pressure of about 2 atm. The patent teaches that avoiding the high stress of normal balloon angioplasty (about 10 atm balloon pressure) reduces side effects such as post operation platelet deposition, clotting, intimal proliferation (scarring) and hormonal changes that cause restenosis. Abele et al. teach that high balloon stress may also cause long term problems including aneurysms (weakening or thinning of the vessel wall). Thermal and/or reduced mechanical injury to the endothelium may well occur with this technique. No mention is made of directing heat towards a particular segment of the artery.
U.S. Pat. No. 5,470,352, issued Nov. 28, 1995, to C. M. Rappaport, discloses a balloon angioplasty device that includes a microwave antenna preferably operating at about 1.8 GHz. The goal of the antenna design is to heat plaque to temperatures in the range of 95.degree. C. to 143.degree. C. without overheating healthy artery tissue by providing heating power in a circumferentially oriented electric field. However the somewhat erroneous presumption is made that plaque is necessarily on the inside of the artery walls rather than normally being within the artery walls where the circumferential field will still apparently heat healthy tissue. As seen above, the applied temperature is quite high. Furthermore, there appears to be no suggestion to direct helical antenna radiation towards a particular radial segment of the artery.
U.S. Pat. No. 5,370,677, issued Dec. 6, 1994 to Rudie et al. discloses a transurethral substantially helical microwave antenna catheter operating at about 915 MHz for thermal treatment of benign prostatic hyperplasmia (BPH). Directional application of heat is accomplished in the range of 45.degree. C. to 60.degree. C. by placing the antenna offset from the axis of the shaft. The heating process takes a period of about 45 minutes, to necrose or ablate, tumorous prostate tissue while a catheter water cooling jacket keeps temperatures adjacent the catheter below about 45.degree. C. to protect adjacent healthy tissue such as the urethra, ejaculatory duct and rectum. The necrosed tissue is reabsorbed by the body. Dilation of a coronary artery is not disclosed.
U.S. Pat. No. 5,359,996, issued Nov. 1, 1994, to L. L. Hood, discloses an ultrasonic cutting tip assembly for an ultrasonic cutting instrument having an ultrasonic transducer.
U.S. Pat. No. 5,199,951, issued Apr. 6, 1993. to J. R. Spears, discloses a balloon angioplasty method for treating a lesion in an arterial wall by bonding a bioprotective material thereon with temperatures in the range of 80.degree. C. to 100.degree. C. for about twenty seconds with another twenty second wait before the balloon is deflated. The bioprotective coating is used to coat the endothelium while it is repairing itself after balloon angioplasty as well as provide drug carriers for the artery to alleviate problems of restenosis and thrombus.
U.S. Pat. No. 5,109,859, issued May 5, 1992, to R. D. Jenkins, discloses a laser ablation catheter system guided by ultrasound sonography to remove atherosclerotic plaque from coronary arteries.
U.S. Pat. No. 5,057,106, issued Oct. 15, 1991, to Kasevich et al., discloses a balloon angioplasty microwave catheter system used for heating arterial plaque. The patent teaches that heating of the plaque reduces restenosis and that the plaque should preferably be heated to about 100.degree. C. for about 30 seconds using a 10 GHz microwave source.
U.S. Pat. No. 4,927,413, issued May 22, 1990, to R. Hess, discloses a flexible shaft for use with balloon angioplasty.
U.S. Pat. No. 4,881,543, issued Nov. 21, 1989, to Trembly et al., discloses a microwave applicator for heating stroma at 2.45 GHz to effect shaping of the cornea with apparatus for cooling of the cornea to protect the endothelium by flow of saline transverse to the antenna axis.
U.S. Pat. No. 4,808,164, issued Feb. 8, 1989, to R. Hess, discloses a catheter for balloon angioplasty that includes a flexible shaft defining a hollow passage.
U.S. Pat. No. 4,700,716, issued Oct. 20, 1987, to Kasevich et al. discloses a microwave antenna for treatment of tumors or other materials by hyperthermia with temperatures of about 50.degree. C. induced with microwave frequency in the range of 500 MHz to 5 GHz. There does not appear to be any provision for protecting endothelial cells from damage.
U.S. Pat. No. 4,685,458, issued Aug. 11, 1987, to M. E. Leckrone, discloses a catheter for use in removing undesired material from a duct with a patients's body including a cutting element, an inflatable bladder, and a pair of abutments to surround the material being removed so that the material is vacuumed out through the catheter.
U.S. Pat. No. 4,643,186, issued Feb. 17, 1987, to Rosen et al., discloses a balloon angioplasty catheter with microwave antenna to heat and soften the plaque by radiation and heat convection. No particular frequencies are provided and no particular precaution is provided for the endothelium.
U.S. Pat. No. 5,129,396, issued Jul. 14, 1992, to Rosen et al., discloses a balloon angioplasty catheter with microwave antenna to measure balloon distension.
U.S. Pat. No. 4,576.177, issued Mar. 18, 1986, to W. W. Webster, Jr., discloses an optical fiber for transmitting laser radiation and an ultrasonic transducer mounted at the tip of the catheter for transmitting and receiving ultrasonic signals.
U.S. Pat. No. 5,150,717, issued Sep. 29, 1992, to Rosen et al., discloses an angioplasty catheter that includes a coaxial transmission line with an elongated center conductor and outer conductor. The transmission frequency is preferably about 3 GHz.
U.S. Pat. No. 4,998,932, issued Mar. 12, 1991, to Rosen et al., discloses a chip generator for either laser or RF radiation located on the distal end of the catheter.
U.S. Pat. No. 5,607,419, issued Mar. 4, 1997, to Amplatz et al., discloses a catheter to apply UV light to a blood vessel after balloon angioplasty. The patent teaches that this treatment retards growth of smooth muscle cells.
I.E.E.E. Transactions on Biomedical Engineering. Vol. BME-34 No. 2, February 1987, by D. M. Sullivan, D. T. Borup, and O. M. P. Gandhi, entitled "Use of Finite Difference Time-Domain Method in Calculating EM Absorption in Human Tissues" describes the FDTD method as applied to bioelectromagnetic problems and demonstrates a 3-D scan of the human torso.
I.E.E.E. Transactions on Biomedical Engineering, Volume 35, No. 4. April 1988, by D. Andreuccetti, M. Bini, A. Ignesti, R. Olmi, N. Rubino, and R. Vanni, entitled "Vee and Polyacrylamide as a Tissue Equivalent Material in the Microwave Range", discloses the use of polyacrylamide gel to simulate biological tissues at microwave frequencies.
Other related references include Critical Reviews in Biomedical Engineering, by K. R. Foster and H. P. Schwan, Volume 17, Issue 1, 1989, entitled "Dielectric Properties of Tissues and Biological Materials" and the book "Field Computation by Moment Methods", by R. F. Harrington, MacMillan Press, 1968.
A review of the above references reveals that a long felt need exists for apparatus and methods to dilate the lumen of the artery without injuring endothelial cells so as to avoid the long term problems often associated with trauma thereto. If the atherosclerotic lesions are radially external of the intima, e.g., in the media, then the intimal layer including the endothelium should be preserved from injury. At a bare minimum, the energy for softening fatty deposits should be radially directed toward the segment or arc of the artery in which the atherosclerotic lesion is located, if the lesion is asymmetrically located within the arterial wall, to greatly reduce the likelihood of damage to healthy tissue. Healthy tissue in the adventitial layer should be preserved. A temperature profile should be controlled to produce heat in a region corresponding to the size and position of the atherosclerotic lesion. The heating should preferably be effected very quickly to avoid extended blockage of the artery with the catheter. Those skilled in the art have long sought and will appreciate the present invention that provides solutions to these and other problems.